It is well known that crosslinking of polymers, e.g. polyolefins, substantially contributes to an improved heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of a polymer. Therefore crosslinked polymers are widely used in different end applications, such as in pipe applications. In a crosslinking reaction of a polymer i.a. interpolymer crosslinks (bridges) are primarily formed. Crosslinking can be effected by radical reaction using i.a. irradiation or free radical generating agents, such as peroxides, which both free radical forming methods are well documented in the literature. An alternative crosslinking technology is e.g. so-called silane crosslinking technology, wherein silane groups are introduced to the polymer by copolymerisation or grafting and the crosslinking is effected by first hydrolysing silane groups which then crosslink in the presence of a crosslinking catalyst as described in patent WO2002096962.
Ethylene polymer is one of the commonly used polymers for crosslinking. It is known that certain properties of ethylene polymer, including properties which can have an effect on the crosslinking efficiency, i.a. on crosslinking rate and degree, may vary i.a. depending on                the type of polymerisation process, such as high pressure polymerisation or a low pressure polymerisation process,        process conditions, and,        especially in case low pressure polymerisation, the catalyst used in the process. For instance polyethylene has typically a characteristic molecular weight distribution (MWD=Mw/Mn), comonomer distribution, so-called long chain branching (LCB) and/or degree of unsaturation depending on the type of the catalyst, such as Ziegler Natta, Cr or single site catalyst, used in polymerisation. Of these variable properties i.a. MWD, and degree of unsaturation may have an effect on the crosslinking efficiency.        
The degree of unsaturation, e.g. the amount of double bonds between two carbon atoms (referred herein as carbon-carbon double bonds), e.g. —CH═CH— or vinyl, i.e. CH2═CH—, moiety, present in the ethylene polymer is known to contribute to the crosslinking efficiency of the ethylene polymer, especially when crosslinking by radical reaction.
Normally, ethylene polymer which is polymerised using Ziegler Natta catalyst has a low degree of unsaturation (typically less than 0.18 vinyls/100° C.). Moreover, typical Ziegler Natta based ethylene polymers are lack of measurable LCB. Therefore ethylene polymer produced using Ziegler Natta catalyst has not been used for crosslinked articles.
Also ethylene polymers produced using conventional single site catalyst have typically a low degree of unsaturation and, furthermore, have usually a narrow MWD which sacrifices the processing of the polymer. Specific single site catalysts are also known in the prior art which produce ethylene polymer with vinyl groups and with Long Chain Branching (LCB). E.g. WO2005103100 of Basell, describes a single site catalyst which provides an ethylene polymer which has more than 0.5 vinyl groups/1000 carbon atoms and also LCB and can be further modified, e.g. crosslinked. Another specific single site catalyst, so called Constrained Geometry Catalyst (CGC), polymerises polyethylene which contains LCB and can be crosslinked, see e.g. in WO9726297 and EP885255 of Dow.
Therefore ethylene polymers produced using Cr catalyst (referred herein as Cr polyethylene), have conventionally been used in crosslinked articles, since Cr catalyst provide relative high degree of unsaturation to the resulting polyethylene (typically more than 0.5 vinyls/100° C.) and are industrially processable.
The current Cr polyethylene has i.a. the drawback that the MWD is very broad. As a consequence the typical Cr polyethylene contains a considerable low molecular weight (Mw) fraction which decreases the mechanical properties, such as strength, of the polymer and also decreases the crosslinking efficiency, since very low molecular weight chains do not provide sufficient crosslinking. The low Mw fraction can also provide inhomogeneities to the Cr polyethylene product which affect adversely to the processability of the polymer and to the quality of the final product. Moreover, the low Mw fraction may cause smoke and fume problems during the processing thereof as well as taste and odour (T&O) problems to the final product. The Cr polyethylene is conventionally produced in a unimodal process which usually limits the tailoring of the Mw and thus MWD.
Low density polyethylene produced in a high pressure process (referred herein as LDPE) is highly branched and can also have beneficial unsaturation for crosslinking. Therefore also LDPE has been used for crosslinking of articles i.a. in applications where “softer” low density polyethylene is desired.
One further known means to increase the unsaturation and thus crosslinking efficiency is to polymerise ethylene together with polyunsaturated comonomers, such as dienes, and/or to add crosslinking promoting agents. However, both means increase the complexity and cost of the production process of the crosslinked article.
There is a continuous need in the polymer field to find alternative polymer solutions suitable for demanding crosslinkable polymer applications, and particularly, where the crosslinked articles must meet high requirements and stringent authority regulations.